Like several of the characters in Harper Lee's novel, To Kill a Mockingbird, Boo is one of the mockingbirds who suffer the indignity of cruelty and hate in the story. As is explained by Atticus early in the story,

“Remember it’s a sin to kill a mockingbird.” That was the only time I ever heard Atticus say it was a sin to do something, and I asked Miss Maudie about it. “Your father’s right,” she said. “Mockingbirds don’t do one thing but make music for us to enjoy... but sing their hearts out for us. That’s why it’s a sin to kill a mockingbird.”

Obviously, Boo does not fly or sing, but like the tiny bird, he is a small, insignificant character who causes no harm to anyone. He is ignorant of things in the outside world, yet he is ridiculed and harassed by those around him--much like the mockingbird would be by someone who decided to kill it. What good would it do? It would only rob the world of a creature that bore no hatred or ill will to others. Both Boo and the mockingbird provide only goodness--the bird by its song and Boo by his protection of the Finch children--in a world where they are subjected to the evils around them.

At the point in the story you are reading (you tagged it with 6-7), the kids are seriously infatuated with the character that is Boo Radley. They are indeed having encounters with him, but at this point in the story, they are afraid of him and what he's about. They believe the rumormill and they let that taint their perspective of him.

The author most directly infers that Boo is innocent every time Atticus (a lawyer, and the hero if this story has one) reprimands the kids for bugging him. Atticus could symbolize the one who is able to look into a soul and determine innocence or guilt. When mockingbirds are discussed, the point of their innocence in addressed. But it takes the reader to put the two together this early in the story at a first read. It will tie together beautifully at the end if you read the whole piece.